In the process wherein a titanium-containing material is subjected to react with sulphuric acid to form a solid reaction mass from which the titanium values are recovered in solution by digestion with water or dilute sulphuric acid leaving behind a reaction residue and the reaction residue is thereafter treated to recover at least part of the titanium contained therein, the improvement wherein the treatment of the reaction residue is effected by contacting with sulphuric acid of a concentration greater than 86% a mixture by weight comprising about 5 to 95% of the reaction residue and 95 to 5% of a slag having a TiO.sub.2 content 60% by weight and a Ti(III) content of about 5 to 40% by weight, and thereafter eventually adding water, steam or dilute sulphuric acid to the mass to start the reaction.